The bed vessel consists of a container which is divided by a bottom into an upper combustion chamber with a fluidized bed and a lower ash discharge chamber for the discharge of ashes and consumed bed material. The bottom may consist of elongated, parallel air distribution chambers with nozzles for combustion air for fluidization of a bed of particulate bed material above the bottom and for combustion of a supplied fuel. The combustion chamber is supplied with combustion air from a compressor. Between the air distribution chambers there are openings through which ashes and bed material may pass from the combustion chamber to the ash chamber below the bed vessel bottom. The ashes and the bed material are cooled by air prior to being discharged via a sluice system.
The chamber for discharging ashes and bed material is usually shaped as a conical or pyramidal hopper with its downwardly-directed tip connected by a tube to the discharge device. In order for the cooling air to be utilized as efficiently as possible, it is introduced at the bottom into the ash discharge chamber and thus flows in the opposite direction to the material to be fed out. It has been found that the cooling air does not spread uniformly over the cross section when flowing upwards through the ash chamber. This means, that the cooling at the outer parts of the ash chamber is unsatisfactory and that the air speed may reach such a level in the central part of the ash chamber that the material in the ash chamber and between air distribution chambers is fluidized. This fluidization increases the heat transfer to the walls of the air distribution chambers and may result in such a heating as to jeopardize the strength. Furthermore, the concentration of the ash cooling air results in an undesired and uneven air distribution over the cross section of the combustion chamber, which may disturb the operation.